The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, a hybrid vehicle refers to a vehicle driven by an internal combustion engine (ICE) using fossil fuel and a motor using electric energy as a driving source.
As a type of a power train of a hybrid vehicle, a transmission mounted electric device (TMED) method of disposing a transmission at an output side of a motor for driving a vehicle has been known.
In a general TMED method, an engine clutch is interposed between an engine and a motor which are driving sources and a transmission is disposed at an output side of the motor so as to transfer complex power of the engine and the motor between which the engine clutch is interposed to a driving wheel through the transmission.
In addition, a TMED hybrid vehicle uses a method in which an engine and a motor are arranged together at a side of a front wheel, a driving wheel so as to permit torques output of the engine and the motor to be overlaid and then to transfer the output to the front wheel through the transmission and, accordingly, only torque transferred to the front wheel is controlled without distribution of driving force to front and rear wheels.
In addition, as another type, a rear-axle mounted electric device (RMED) method has been known and an RMED hybrid vehicle is configured to drive a front wheel by engine power and to drive a rear wheel by motor power.
The RMED method employs a four-wheel drive (4WD) method and U.S. Pat. No. 7,517,298 discloses an electronic-4WD (e-4WD) method of applying a motor to a rear wheel without a propeller shaft, a mechanical power device in order to enhance efficiency of a 4WD vehicle.
As such, a hybrid vehicle with an e-4WD system uses independent drivers that are applied to a front wheel and a rear wheel, respectively and, in this regard, an engine is applied to a driver of the front wheel, a motor is applied to a driver of the rear wheel, and the drivers are independently or simultaneously driven under a condition of a driving environment.
For example, it may be possible to perform switching between a two-wheel drive (2WD) mode and a 4WD mode, and only the front wheels function as driving wheels in the 2WD mode but both the front wheels and the rear wheels function as driving wheels because driving force is distributed to the front wheels and the rear wheels in the 4WD mode.
In the 4WD mode, since driving force is distributed to the front wheels and the rear wheels, torque transferred to the rear wheels as well as the front wheels is controlled and, in this case, torque is distributed to the front wheels and the rear wheels so as to satisfy total torque requirement required for vehicle driving.
Hereinafter, according to a result of distribution of driving force to the front wheels and the rear wheels, torque applied to the front wheels from a driving source of a vehicle will be referred to as front wheel torque or torque of front wheel and torque applied to the rear wheels will be referred to rear wheel torque or torque of rear wheel.
Accordingly, in order to satisfy the total torque requirement required for vehicle driving, that is, driver torque requirement, the front wheel torque and the rear wheel torque are distributed and controlled and, in the above e-4WD system, torque output from a motor is controlled in order to control the rear wheel torque.
The e-4WD system is configured to differentially distribute driving force of front and rear wheels according to driving situation so as to enhance driving stability.
In general, control of a 4WD system includes slip control for improved distribution of driving force to front/rear wheels in order to provide normal startup, acceleration, and hill-climbing capabilities when there is a speed difference between the front wheels and the rear wheels during vehicle startup or acceleration, particularly, hill climbing on a slippery road and handling control for optimal distribution of driving force to front/rear wheels in order to inhibit understeer or oversteer during cornering and to ensure normal cornering.
As is well known, control strategies for providing driving stability of a vehicle are largely configured to make dynamic characteristics of respective driving, braking, steering, and suspension systems of a vehicle to satisfy target performance based on the driving, braking, steering, and suspension systems.
The control strategies are independently configured, priority based on stability is allocated according to a driving condition and a running condition of a vehicle, and the vehicle sequentially enters corresponding control with the allocated priority.
4WD slip control and handling control are also managed according to cooperative control strategy similar to the above strategy and, in general, a vehicle preferentially enters handling control compared with slip control and is driven in corresponding control.
However, there is a limit in providing stability in a complex driving situation (e.g., slip startup on curved road)) using an on/off method of single control. When there is frequent transition between control modes, response performance in a transient period may be affected.